What are coherent sources and incoherent sources?

(N/A) Coherent sources: Two sources of light are said to be coherent if they emit light waves of the same frequency and maintain a constant phase difference with respect to time. These sources are necessary to observe a stable interference pattern.
Incoherent sources: Two sources of light are said to be incoherent if they emit light waves of different frequencies or if they emit light waves of the same frequency but with a phase difference that changes randomly and rapidly with time. These sources do not produce a stable interference pattern.